The present invention relates to a method and apparatus for measuring the permeability of certain barrier materials to any gas. More particularly, a preferred embodiment of the invention relates specifically to a method for measuring the oxygen permeability of a flat sample barrier film material.
Certain apparatus for measuring the oxygen flow rate through a membrane barrier material are known in the prior art. For example, U.S. Pat. No. 3,618,361, issued Nov. 9, 1971, discloses an early system for measuring the gas permeability of a film. Similarly, U.S. Pat. No. 3,590,634, issued Jul. 6, 1971, discloses another instrument for measuring permeation rates through a membrane. U.S. Pat. No. 4,467,927, issued Aug. 14, 1984, discloses an apparatus for measuring gas transmission through films in multiple test cells. All of these devices operate in conjunction with an oxygen detector which typically provides an electrochemical transformation in response to the presence of oxygen. One such oxygen detector is disclosed in U.S. Pat. No. 3,223,597, issued Dec. 14, 1965, and another form of oxygen detector is disclosed in U.S. Pat. No. 4,085,024, issued Apr. 18, 1978. All of these earlier patents, and a considerable number of more recent patents, utilize a test cell setup in conjunction with an oxygen detector to derive an electrical signal which is representative of the amount of oxygen with in a given chamber of the test cell. A sample of the barrier material undergoing test is typically clamped within the test cell to form two chambers, wherein one chamber is initially free of oxygen and filled with a neutral gas such as nitrogen, and the other chamber is initially saturated with oxygen. Before proceeding with these initial conditions, it is first necessary to outgas all oxygen from the material sample undergoing tests. Outgasing is accomplished by flowing a neutral gas such as nitrogen through both chambers described above, monitoring the test gas for oxygen content until it appears that the oxygen content has become depleted to zero, or near zero, and then proceeding with the initial conditions described above. The test process requires that the neutral test gas flow be monitored until the oxygen concentration in the test gas reaches a steady state level, which can require many hours of operation. In general, the amount of time required for such a test is directly related to the permeability coefficient of the material and to the material thickness. The permeability coefficient is directly related to temperature and, to a lesser extent, pressure. The objective of tests of this type is to measure the amount of oxygen which permeates through the test membrane under steady state conditions, and the oxygen measurements are typically made by devices such as are disclosed in the foregoing prior art patents.
The large majority of permeation measurements now being made are in terms of the amount of gas permeating a given sample. This may be a container or an essentially flat sample. The answers are given in terms of the volume or weight of a gas permeating the sample in a given time. In the case of a container, this becomes the volume or weight of gas per time per container. In the case of a flat sample, it is the volume or weight of a gas per time per unit area. These answers are obtained and referred to the conditions of the test. In a formal way, these are not permeation values but are transmission rate values for that gas, through that sample under the specific test conditions.
For example: ##EQU1##
The definition of the permeation rate for a film (in the same units) is referred to a standard temperature and pressure (STP) (760 mm Hg, 0.degree. C.) for a 1 mil film. The amount of gas being transferred is roughly inversely proportional to the film thickness. At the test conditions, a 1 mil film would then transmit ten times as much O.sub.2 as a 10 mil sample. ##EQU2## Correction to 0.degree. C. would then result in the formally defined permeation rate.
Basically, the permeation of gas through a material results from the inherent physical characteristics of the material. These characteristics have been formally defined in all of the literature in the field for the last 30 years. These characteristics are: the solubility of the gas of interest in the material and the rate of diffusion of the gas through the material. The solubility is the volume of gas which will dissolve in a like volume of the material ##EQU3## and the diffusion coefficient denotes the rate at which the gas moves through the material The product of the solubility coefficient and the diffusion coefficient is called the permeability coefficient; and in this case, the units are thus: ##EQU4## The relationship between P and P ms simply a unit conversion factor which is EQU Conversion factor=2.94.times.10.sup.-12
so that: ##EQU5##
As noted above, this information is well known. All aspects have been reviewed for years in the literature on the subject of permeation. The background is necessary, however, to follow the new method of measurement of the transmission of gas through a material.
The most used present method of measurement today is termed isostatic. This refers to the case in which a sample is mounted in such a way that one side of the sample is exposed to the gas of interest. The other side is isolated at zero, or extremely low levels, of that gas. In this way, the gas permeating the sample can be measured as a function of time.
Usually the film is first outgased by flowing a neutral gas over both sides of the sample. Then the permeant gas is made to flow on one side. The final answer is obtained by waiting until the permeant gas level on the sensor side reaches a steady state value. These times become quite long for even moderately good barriers. For instance, a PET film, 10 mil (10.sup.-2 inches) thick, at 30.degree. C., has a transmission value of approximately 7.5 cc/M.sup.2 .multidot.day. Many barriers today are at least one-tenth of this value. Even so, the outgasing for the 10 mil sample will take about 21 hours; and the permeation measurement requires about 29 hours.
It would be extremely desirable if the amount of time required for making valid permeation measurements could be significantly reduced. The equipment required for making such measurements is fairly expensive and complex; and therefore, the measurement of a single sample of material can require the exclusive use of such equipment for a period of several days. If a significant number of samples require measurement, the number of test stations set up with the necessary equipment for such measurements must be multiplied to fit the testing schedule. Therefore, any modification through the overall process which can be made by way of shortening the total test time will be of great advantage and significance in the field.